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EQUIPMENT CLEANING | PROCESSING


problems that can arise from this, however, are often overlooked. Uncontrolled development of heat poses a high risk to expensive tooling. The subsequent costs for repair or new procurement often show this to be anything but a low cost option. In addition, the gases created pose a risk to the operator and the environment.


Thermal cleaning The optimal cleaning method must ensure perfect cleaning of the parts – externally and internally – while also protecting the components from mechanical or thermal damage and avoiding environmental problems. Thermal cleaning systems are able to fulfil all these aspects. Large gas-fired cleaning ovens (such as the Schwing Maxiclean) are well suited for cleaning larger components, such as pelletising heads, and leave no residues. They offer an automated process that can be adapted to suit the item to be cleaned and the amount of polymer to be removed. Parts to be cleaned are loaded onto a cart at


room temperature. After the door of the oven is closed, heating of the thermal afterburner and the oven is started and the adhered plastic contamina- tion is slowly carbonised. Temperature control of the oven prevents any thermal damage occurring to the metal parts while the carbonisation gases that are formed enter a separate downstream afterburning unit. The oven chamber is slowly cooled down at the end of the cleaning process. An entire cleaning cycle (without cooling time) takes around four to five hours. Vacuum pyrolysis systems (such as Schwing’s


Vacuclean) comprise an electrically-heated chamber ovens with a catalytic afterburner. The advantages of this system include more precise control of temperature through direct measure- ment of the workpiece as well as a slower and more gentle heat-up of the parts. The Vacuclean system is suitable for parts up to six metres long. Parts to be cleaned are loaded into a chamber


at room temperature in a basket. Once the lid is closed, the cleaning process is commenced. A vacuum pump extracts the air from the oven chamber (to around 220 millibar) and this reduced atmosphere causes pyrolysis and some oxidation, which removes organic contaminations. The low-temperature carbonisation gases are cleaned in the downstream catalytic converter. The entire cleaning cycle lasts around 8 to 16 hours.


www.compoundingworld.com


Fluidised bed Where a particularly short cleaning time is required, fluidised bed technology can be used (Schwing has its own Innova- clean system). Adhered plastic contaminants are carbonised through thermal-oxidative decom- position. At the bed temperatures of used (400°C to 480°C and up to 520°C in special cases) only inorganic pigments and filler media remain.


Fluidised bed systems are suitable for all polymers, including PVC, PTFE and PEEK, and can handle assembled and disassembled tools. Other advantages include short cleaning times with no mechanical (abrasive) or thermal damage to the tools and without carbon residues. Operation is simple. The system is


preheated to the required operating temperature by an electric heater and the parts to be cleaned loaded directly into the hot fluidised bed in a basket. Thermal-oxidative decomposition takes place through the atmospheric oxygen of the fluidised air. Cleaning time is typically from one to two hours, depending on the size of the part. Generated off-gas is directed to the thermal afterburner and in special cases (such as for halogenated polymers) it is possible to install a scrubber.


Contract cleaning


All compounding equipment parts can be cleaned using thermal methods. Where there is insufficient capacity to justify an in-house system, a contract cleaning company can offer a suitable solution. The decision whether a user performs their own cleaning or contracts the process out to an external company depends on the individual situation. � www.schwing-technologies.com


About the author


Udo Heffungs is Senior Expert at Schwing Technologies. Based in Germany, Schwing has been actively involved in thermal cleaning for almost 50 years. The company manufactures its own range of thermal cleaning systems. It also offers contract cleaning services from seven locations worldwide and currently handles up to 250,000 parts annually.


October 2018 | COMPOUNDING WORLD 61


Above and left: Before and after images showing a pelletising plate that has been cleaned at Schwing Image: Schwing Technologies


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